Optical filter and optical device provided with this optical filter

ABSTRACT

Light emitted from a taking lens  20  enters a first birefringent plate  1   a  to be spatially divided along a first direction extending perpendicular to the direction in which the light advances to achieve two separate rays L 10  and L 20 . The vibrational planes of the two light fluxes L 10  and L 20  emitted from the first birefringent plate  1   a  are converted to a circularly polarized light by a phase plate  1   c . The two light fluxes L 10 ′ and L 20 ′ emitted from the phase plate  1   c  are each spatially divided into two by a second birefringent plate id along a second direction extending perpendicular to the first direction to achieve four separate rays L 11 , L 12 , L 21  and L 22 , to be guided to an imaging plane  15   a  of an imaging device  15 . At least either the first birefringent plate or the second birefringent plate is constituted of lithium niobate, rutile, Chilean nitrate, or the like.

This is a Divisional of application Ser. No. 10/653,223, filed Sep. 3,2003, which in turn is a Divisional of application Ser. No. 10/119,702,filed Apr. 11, 2002 (now U.S. Pat. No. 6,778,325), which is a Divisionalof application Ser. No. 09/772,931, filed Jan. 31, 2001 (now U.S. Pat.No. 6,392,803), which is a Divisional of application Ser. No.09/281,324, filed Mar. 30, 1999 (now U.S. Pat. No. 6,327,085). Theentire disclosures of the prior applications are hereby incorporated byreference herein in their entireties.

INCORPORATION BY REFERENCE

The disclosures of the following priority applications are hereinincorporated by reference:

Japanese Patent Application No. 10-101822, filed Mar. 31, 1998

Japanese Patent Application No. 10-197610, filed Jul. 13, 1998

Japanese Patent Application No. 11-18596, filed Jan. 27, 1999

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical filter and an optical deviceprovided with this optical filter.

2. Description of the Related Art

In a digital still camera employing an imaging device such as a CCD(hereafter a digital still camera is simply referred to as a “DSC” inthis specification), “beat” interference may occur as a result of acertain relationship between the spatial frequency of the subject imageand the repetitive pitch of dot-type on-chip color separation filtersprovided at the front surface of the imaging device. In order to preventany false color signals from being generated by the beat, i.e., in orderto prevent the so-called “color moire,” an optical low-pass filter isprovided between the taking lens and the imaging device. The opticallow-pass filter, which is constituted by employing a birefringent plateachieving birefringence, reduces the generation of the beat through thebirefringent effect provided by the birefringent plate. Normally, quartzis employed to constitute the birefringent plate.

Japanese Examined Patent Publication No. 1994-20316 proposes an opticallow-pass filter employing two birefringent plates such as that describedabove, which is suited for application in an imaging device providedwith dot-type on-chip color separation filters. This optical low-passfilter is constituted by enclosing a quarter-wave plate between twobirefringent plates with the directions in which the image becomesshifted through the birefringence offset by approximately 90° from eachother.

Now, the so-called direct image forming system, in which the imagingdevice is directly provided at the primary image forming plane of thetaking lens without employing a reduction lens system or the like isbecoming the mainstay in single lens reflex type DSCs that allowinterchange of the taking lens among DSCs in recent years. The advent ofthe direct image forming system has been realized through theutilization of imaging devices having a large image area ofapproximately 15.5 mm×22.8 mm that have been manufactured in recentyears to replace ⅔″ size (approximately 6.8 mm×8.8 mm) and 1″ size(approximately 9.3 mm×14 mm) imaging devices that have beenconventionally used in television cameras and the like. With this sizeof image area available, an image plane having a size (approximateaspect ratio 2:3=15.6 mm×22.3 mm), which is comparable to the imageplane size of the C-type silver halide film IX240 system (APS), isachieved. By employing an imaging device achieving a relatively largeimage area, it becomes possible to adopt a camera system that employsthe 135-type photographic film in a DSC. To explain this point,providing a ⅔″ size or 1″ size imaging device at the field of a camerausing the 135-type film only achieves a small image plane size for theimaging device compared to the image plane size of the 135-type film (24mm×36 mm). As a result, a large difference will manifest in the angle offield achieved by a taking lens having a specific focal length, to causethe photographer to feel restricted. This problem becomes eliminated asthe image area of the imaging device increases and becomes closer to theimage plane size of the 135-type film.

However, as the image area in a single lens reflex type DSC, which formsthe primary image with the taking lens at an image device directly,increases, the problems explained below arise to a degree to which theycannot be neglected.

Imaging devices in DSCs in recent years have evolved in two directions,i.e., toward a higher concentration of pixels and toward a larger imageplane. When the number of pixels is increased to exceed 1 million pixelswhile maintaining the size of the image plane at approximately ⅓″ to ½″as in the prior art, the pixel pitch becomes reduced. For instance, inan imaging device having approximately 1,300,000 pixels, with its imageplane size at approximately ⅓,″ the pixel pitch is approximately 4 μm.Generally speaking, the pixel pitch “p” at an imaging device and thethickness “t” of the birefringent plates constituting the opticallow-pass filter which is employed to support the pixel pitch “p” achievethe relationship expressed through the following equation (1)p=t(ne ² −no ²)/(2ne×no)  (1)witht: birefringent plate thicknessne: extraordinary ray refractive index at birefringent plateno: ordinary ray refractive index at birefringent plate

When a quartz plate, which is most commonly employed to constitute abirefringent plate, is used in an imaging device with a pixel pitch ofapproximately 4 μm, the thickness “t” required of the quartz plate isconcluded to be approximately 0.7 mm by working backward with “p” inequation (1) set at 4 μm, since the refractive indices of quartz forlight having a wavelength of 589 nm are ne=1.55336 and no=1.54425. Sincethe thickness of the quarter-wave plate needs to be approximately 0.5 mmregardless of the pixel pitch p, the entire thickness achieved whenconstituting an optical low-pass filter by pasting together threeplates, i.e., two quartz plates (birefringent plates) and onequarter-wave plate, will be approximately 2 mm.

However, when the area of the photosensitive surface of an imagingdevice increases, as in the case of, in particular, an imaging deviceemployed in a single lens reflex type DSC, it becomes necessary toincrease the thickness of the optical low-pass filter for the reasonsdetailed below.

While the size of the image plane of a ⅓″ imaging device isapproximately 3.6 mm×4.8 mm, let us now consider an imaging devicehaving an image plane size equivalent to that of the C-type (aspectratio 2:3=16 mm×24 mm) in an IX240 system (advanced photo system (APS))with silver halide film. When pixels are arrayed at a pixel pitch ofapproximately 4 μm on this imaging device, the total number of pixelsfor the entire image plane will exceed 20 million by simple calculation,and it is considered that the current technical level is not high enoughto realize such a large number of pixels for practical use from theviewpoints of the yield in imaging device production, the scale andprocessing speed of the image information processing circuit and thelike. As a result, it is assumed that it is appropriate to set thenumber of pixels at approximately two million and several hundreds ofthousands in an imaging device having a large image plane equivalent tothat of the APS-C type, which sets the pixel pitch at 10 and several μm.

For instance, when an APS-C size imaging device (16 mm×24 mm) isprepared at a pixel pitch set to 12 μm, the number of pixels in theimaging device will be approximately 2,670,000. When constituting thebirefringent plates of the optical low-pass filter employed incombination with the imaging device having the pixel pitch of 12 μm withquartz, the thickness of a single quartz plate is calculated to be“t=2.04 mm by incorporating “p”=12 μm in equation (1). By adding thethicknesses of two such quartz plates and a quarter-wave plate (0.5 mm),the thickness of the optical low-pass filter is calculated to be 4.58mm, which is more than twice as large as the thickness of an opticallow-pass filter (thickness: 2 mm) with the pixel pitch set at 4 μm.

In addition, since the spectral sensitivity of an imaging device isdifferent from the spectral sensitivity of the human eye, an IR blockingfilter is normally provided to cut off infrared light within the imagingoptical path in a DSC employing an imaging device. This IR blockingfilter (thickness; approximately 0.8 mm) is also provided pasted to theoptical low-pass filter. Thus, the entire thickness of the opticallow-pass filter supporting the pixel pitch of 12 μm will go up to 5.38mm when the thickness of the IR blocking filter is included.

It is difficult to place an optical low-pass filter having such athickness between a taking lens and an imaging device. Even in the caseof a regular lens shutter type DSC, which does not require any member tobe provided between the rear end of the taking lens and thephotosensitive surface of the imaging device except for the opticallow-pass filter, it must be ensured in design that the minimum value(the so-called back focal distance) of the distance between the rearmostend of the taking lens and the imaging device is larger than thethickness of the optical low-pass filter. Setting the length of the backfocal distance of the taking lens larger than the focal length of thetaking lens imposes restrictions in terms of the optical design.

Furthermore, in the case of a single lens reflex type DSC, whichdirectly forms an image of the subject achieved by a taking lens on alarge size imaging device without employing a reduction lens system, aquick return mirror for switching the optical path between theviewfinder and the imaging system or a fixed semitransparent mirror(beam splitter) is needed between the taking lens and the imagingdevice. In addition, a mechanical shutter is required for defining anexposure time and for blocking the imaging device from exposure duringan image signal read operation at the imaging device. While thisstructure having a mirror and a shutter provided between the taking lensand its image forming plane is also adopted in a single lens reflexcamera that employs regular silver halide film, it is difficult toprovide an optical low-pass filter having a thickness exceeding 5 mm inaddition while ensuring that it does not present any obstacle in theoperation or the mirror at the shutter. It merits particular note thatmore and more cameras in recent years adopt the autofocus (AF) function,and that a single lens reflex camera with the AF function adopts astructure having a sub mirror provided to the rear of the quick returnmirror, i.e., between the quick return mirror and the shutter, to guidelight flux to a focal point detection device. This makes it even moredifficult to position an optical low-pass filter having a thicknessexceeding 5 mm.

In addition to the problem of an increased thickness of the opticallow-pass filter resulting from a larger pixel pitch in a larger imagingdevice as described above, another problem arises as detailed below.

Normally, the length of the air equivalent optical path achieved whenlight is transmitted and advances through a medium having a thickness“t” and a refractive index “n” is expressed as t/n. In other words, theair equivalent optical path lengths achieved when light advances throughmedia having the same refractive index “n” but having differentthicknesses “t”, vary. Now, let us consider light emitted from one pointon the optical axis of a photographic optical system toward an imagingdevice to reach the center of the image plane of the imaging device andlight emitted from the same point on the optical axis of thephotographic optical system toward the imaging device to reach theperiphery of the image plane.

Since the light that reaches the center of the image plane enters thelight entry surface of the optical low-pass filter at almost a rightangle, “t” roughly equals the thickness of the optical low-pass filter.In contrast, since the light reaching the periphery of the image planeadvances diagonally through the optical low-pass filter, “t” here islarger than the “t” encountered by the light reaching the center of theimage plane. Since the lengths of the air equivalent optical pathsachieved by light being transmitted through the optical low-pass filterare different for the light reaching the center of the image plane andthe light reaching the periphery of the image plane as explained above,a focus misalignment occurs in the direction of the optical axis betweenthe image plane center and the image plane periphery. The degree of thisfocus misalignment increases as the thickness of the optical low-passfilter increases as described above, which may result in a reduced imagequality at the peripheral area of the image plane.

As the size of the imaging device is increased, a problem of foreignmatter becoming transferred as explained next, i.e., a problem offoreign matter such as dust and lint adhering to the photosensitivesurface of the imaging device to cast a shadow onto the image capturedby the imaging device, tends to occur readily, in addition to theproblems discussed above. In particular, in an interchangeable lens typeDSC in which foreign matter such as dust and lint readily enters themirror box when the taking lens is detached, this problem is morepronounced.

A similar problem occurs in optical devices such as facsimile machinesand image scanners when foreign matter such as dust and lint materializeas a document is transmitted or the document read unit moves, which maybecome adhered to the vicinity of the photosensitive surface of thephotoelectric conversion element or the glass (platen glass) upon whichthe document is placed to result in a shadow being cast on the inputimage, as in the interchangeable lens type DSC.

Now, since the crystal of quartz employed to constitute birefringentplates imparts a piezoelectric effect, the crystal itself is caused tobecome electrically charged readily by vibration or the like. The quartzcrystal also has a property that does not allow a stored electricalcharge to be discharged easily. In addition, since an insulatingmaterial such as plastic, ceramic or the like is employed to constitutethe imaging device package, the electrical charge stored at the imagingdevice cannot be released with ease.

Vibration and air currents occurring as a result of an operation of anoptical device sometimes cause the foreign matter discussed above tobecome suspended inside the optical device, which may ultimately becomeadhered to the electrically charged birefringent plates, imaging deviceor the like, as explained above. Consequently, the operator of theoptical device is required to clean the optical device frequently toprevent shadows from being cast as explained earlier.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an optical filterachieving a small thickness and an optical device provided with theoptical filter.

A second object of the present invention is to provide an opticallow-pass filter which is capable of preventing the loss of image qualityat the periphery of the image plane even when the image area at theimaging device is expanded or even when the pixel pitch is increased,and an optical device provided with the optical low-pass filter.

A third object of the present invention is to prevent foreign matterfrom becoming adhered to the optical filter described above, aphotoelectric conversion element and the like to cast shadows thereupon,by neutralizing an electrical charge occurring as a result of theoptical filter, the photoelectric conversion element and the likebecoming electrically charged.

In order to achieve the objects described above, the present inventioncomprises a first birefringent plate constituting an optical elementthat spatially divides incident light into two separate light fluxesalong a first direction extending perpendicular to the direction inwhich the incident light advances, a vibrational plane converting platethat changes the vibrational planes of the two light fluxes emitted fromthe first birefringent plate and a second birefringent plateconstituting an optical element that spatially divides each of the twolight fluxes emitted from the vibrational plane converting plate intotwo light fluxes along a second direction that is different from thefirst direction to achieve a total of four separate light fluxes, withat least either the first birefringent plate or the second birefringentplate, constituted of a material having a larger difference between theextraordinary ray refractive index and the ordinary ray refractive indexcompared to that of quartz.

In addition, according to the present invention, an anti-reflectioncoating is applied to a boundary surface of the first birefringent plateand an optical element provided adjacent to the first birefringent plateand a boundary surface of the second birefringent plate and an opticalelement provided adjacent to the second birefringent plate.

Furthermore, according to the present invention, the vibrational planeconverting plate is constituted of a phase plate that is capable ofcreating a phase difference of a specific quantity between a lightcomponent that vibrates in one vibrating direction and a light componentthat vibrates in another vibrating direction extending perpendicular tothe one vibrating direction for each of the two light fluxes emittedfrom the first birefringent plate.

According to the present invention, the vibrational plane convertingplate may be constituted of an optical rotatory plate provided as anoptical element that rotates the directions of vibration of the twolight fluxes emitted from the first birefringent plate at thevibrational plane by a specific degree.

Alternatively, the present invention comprises a first birefringentplate for spatially dividing light emitted from an image forming lensalong a first direction to achieve two separate light fluxes, a phaseplate that creates a phase difference of a specific quantity between alight component that vibrates in one vibrating direction and a lightcomponent that vibrates in another vibrating direction extendingperpendicular to the one vibrating direction for each of the two lightfluxes emitted from the first birefringent plate and a secondbirefringent plate having almost the same thickness and almost the samerefractive index as those of the first birefringent plate, provided forspatially dividing each of the two light fluxes emitted from the phaseplate along a second direction that is different from the firstdirection to achieve two separate light fluxes to be guided to theimaging plane of the imaging device, with the thickness t1 and therefractive index n1 of the first birefringent plate and the secondbirefringent plate satisfying the following conditional equation, with Arepresenting the image height at the image plane corners, POrepresenting the air equivalent optical path length extending from theimaging plane to the exit pupil of the image forming lens and A/PO≧0.15satisfied. $\begin{matrix}{{{t1} \leqq {C \times \frac{n1}{Y({n1})}}}{with}} & (2) \\{{Y({n1})} = {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}}} & (3) \\{C = {\frac{1}{2} \times \left\{ {{K \times B \times d \times {Fno}} - {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right) \times \frac{t2}{n2}}} \right\}}} & (4) \\{{\sin\quad\theta\quad 1} = \frac{\sin\quad\phi}{n1}} & (5) \\{{\theta\quad 1} = {\sin^{- 1}\left( \frac{\sin\quad\phi}{n1} \right)}} & (6) \\{{\theta\quad 2} = {\sin^{- 1}\left( \frac{{n1} \times \sin\quad\phi}{n2} \right)}} & (7) \\{0.25 \leqq K \leqq 0.35} & (8) \\{1 \leqq B \leqq 3} & (9)\end{matrix}$

-   -   t1: thicknesses of the first birefringent plate and the second        birefringent plate    -   n1: refractive indices of the first birefringent plate and the        second birefringent plate    -   t2: thickness of the phase plate    -   n2: refractive index of phase plate    -   d: pixel pitch at the imaging device    -   φ: Angle of incidence at a first birefringent plate of light        flux entering corner of the imaging plane of the imaging device        from the center of the exit pupil of the taking lens    -   Fno: F number of the taking lens

Alternatively, the present invention may comprise a first birefringentplate for spatially dividing light emitted from an image forming lensalong a first direction to achieve two separate light fluxes, a phaseplate that creates a phase difference of a specific quantity between alight component that vibrates in one vibrating direction and a lightcomponent that vibrates in another vibrating direction extendingperpendicular to the one vibrating direction for each of the two lightfluxes emitted from the first birefringent plate and a secondbirefringent plate having almost the same refractive index as that ofthe first birefringent plate, provided for spatially dividing each ofthe two light fluxes emitted from the phase plate along a seconddirection that is different from the first direction to achieve twoseparate light fluxes to be guided to the imaging plane of the imagingdevice, with the thickness t11 and the refractive index n1 of the firstbirefringent plate and the thickness t12 and the refractive index n1 ofthe second birefringent plate satisfying the following conditionalequation, with A representing the image height at the image planecorners, PO representing the air equivalent optical path lengthextending from the imaging plane to the exit pupil of the image forminglens and A/PO≧0.15 satisfied. $\begin{matrix}{{{{t11} + {t12}} \leqq {{C1} \times \frac{n1}{Y({n1})}}}{with}} & (10) \\{{Y({n1})} = {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}}} & (11) \\{{C1} = {{K \times B \times d \times {Fno}} - {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right) \times \frac{t2}{n2}}}} & (12) \\{{\sin\quad\theta\quad 1} = \frac{\sin\quad\phi}{n1}} & (13) \\{{\theta\quad 1} = {\sin^{- 1}\left( \frac{\sin\quad\phi}{n1} \right)}} & (14) \\{{\theta\quad 2} = {\sin^{- 1}\left( \frac{{n1} \times \sin\quad\phi}{n2} \right)}} & (15) \\{0.25 \leqq K \leqq 0.35} & (16) \\{1 \leqq B \leqq 3} & (17)\end{matrix}$

-   -   t11: thickness of the first birefringent plate    -   t12: thickness of the second birefringent plate    -   n1: refractive indices of the first birefringent plate and the        second birefringent plate    -   t2: thickness of the phase plate    -   n2: refractive index of phase plate    -   d: pixel pitch at the imaging device    -   φ: Angle of incidence at a first birefringent plate of light        flux entering corner of the imaging plane of the imaging device        from the center of the exit pupil of the taking lens    -   Fno: F number of the taking lens

Alternatively, the present invention may comprise a first birefringentplate for spatially dividing light emitted from an image forming lensalong a first direction to achieve two separate light fluxes, a phaseplate that creates a phase difference of a specific quantity between alight component that vibrates in one vibrating direction and a lightcomponent that vibrates in another vibrating direction extendingperpendicular to the one vibrating direction for each of the two lightfluxes emitted from the first birefringent plate and a secondbirefringent plate having a different thickness and a differentrefractive index from those of the first birefringent plate, providedfor spatially dividing each of the two light fluxes emitted from thephase plate along a second direction that is different from the firstdirection to achieve two separate light fluxes to be guided to theimaging plane of the imaging device, with the thickness t11 and therefractive index nil of the first birefringent plate and the thicknesst12 and the refractive index n12 of the second birefringent platesatisfying the following conditional equation, with A representing theimage height at the image plane corners, PO representing the airequivalent optical path length extending from the imaging plane to theexit pupil of the image forming lens and A/PO≧0.15 satisfied.$\begin{matrix}{{{{\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}} \right) \times \frac{t11}{n11}} + {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 3}} \right) \times \frac{t12}{n12}}} \leqq {C2}}{with}} & (18) \\{{C2} = {{K \times B \times d \times {Fno}} - {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right) \times \frac{t2}{n2}}}} & (19) \\{{\sin\quad\theta\quad 1} = \frac{\sin\quad\phi}{n11}} & (20) \\{{\theta\quad 1} = {\sin^{- 1}\left( \frac{\sin\quad\phi}{n11} \right)}} & (21) \\{{\theta\quad 2} = {\sin^{- 1}\left( \frac{{n11} \times \sin\quad\theta\quad 1}{n2} \right)}} & (22) \\{{\theta\quad 3} = {\sin^{- 1}\left( \frac{{n2} \times \sin\quad\theta\quad 2}{n12} \right)}} & (23) \\{0.25 \leqq K \leqq 0.35} & (24) \\{1 \leqq B \leqq 3} & (25)\end{matrix}$

-   -   t11: thickness of the first birefringent plate    -   t12: thickness of the second birefringent plate    -   n11: refractive index of the first birefringent plate    -   n12: refractive index of the second birefringent plate    -   t2 thickness of the phase plate    -   n2: refractive index of phase plate    -   d: pixel pitch at the imaging device    -   φ: Angle of incidence at a first birefringent plate of light        flux entering corner of the imaging plane of the imaging device        from the center of the exit pupil of the taking lens    -   Fno: F number of the taking lens

The present invention is further provided with a neutralizing circuitfor neutralizing electrical charges stored at the first birefringentplate and the second birefringent plate.

In addition, the present invention is provided with a neutralizingcircuit for neutralizing at least one of the electrical charges storedat the optical filter, the image forming lens and the imaging device.

The present invention is provided with a photoelectric conversionelement for converting an optical image guided to a photosensitiveportion of the photoelectric conversion element to an electrical signal,having a cover member covering the photosensitive portion, a transparentelectrode formed at a front surface of the cover member and a conductivecircuit electrically connected with the transparent electrode andprovided to neutralize any electrical charge occurring at thephotoelectric conversion element caused by the operation of theelectrical system.

Furthermore, the present invention is provided with a photoelectricconversion element for converting an optical image formed by an imageforming lens to an electrical signal, an optical member provided in anoptical path between the image forming lens and the photoelectricconversion element, a transparent electrode provided, at least, at asurface of an optical member located in the vicinity of the imageforming plane of the image forming lens and a conductive memberelectrically connected with the transparent electrode and provided forneutralizing the electrical charge occurring at the optical member.

The present invention may be further provided with a voltage source thatreduces the force with which matter adhering to the photoelectricconversion element by applying a voltage to a conductive member.

The present invention is further provided with a shutter that can beswitched between a light blocking state in which a light flux enteringthe photoelectric conversion element is blocked and an open state inwhich the light flux is allowed to pass, with the conductive circuitprovided to neutralize the electrical charge occurring at thephotoelectric conversion element as a result of a shutter operation.

In addition, the present invention may be further provided with avoltage source that reduces the force with which foreign matter adheresto the optical member by applying a voltage to the conductive member.

The present invention is further provided with a control circuit thatsustains the open state of the shutter and applies the voltage to theoptical member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of the optical filter according to thepresent invention;

FIG. 2 is a schematic illustration of the structure and the principle ofthe optical filter according to the present invention;

FIG. 3 is a longitudinal sectional view illustrating an example of thestructure of a single lens reflex type digital still camera providedwith the optical filter according to the present invention;

FIG. 4 illustrates the relationship between the thickness of the opticalfilter and the pixel pitch at the imaging device;

FIG. 5 is a longitudinal sectional view illustrating another example ofthe structure of a single lens reflex type digital still camera providedwith the optical filter according to the present invention;

FIG. 6 presents a schematic structure of the optical filter in anembodiment of the present invention, illustrating an example of thecombination of two birefringent plates having equal thicknesses andrefractive indices;

FIG. 7 illustrates focus misalignment caused by an optical low-passfilter;

FIG. 8 illustrates the relationship between the angle of incidence of aray of light entering the optical low-pass filter and the focusmisalignment quantity;

FIG. 9 illustrates the relationship between the angle of incidence of aray of light entering the optical low-pass filter and the focusmisalignment quantity;

FIG. 10 illustrates a method for selecting the combination of therefractive indices and the thicknesses of the birefringent plates;

FIG. 11 illustrates another method for selecting the combination of therefractive indices and the thicknesses of the birefringent plates;

FIG. 12 illustrates yet another method for selecting the combination ofthe refractive indices and the thicknesses of the birefringent plates;

FIG. 13 presents a schematic structure of the optical low-pass filter inan embodiment of the present invention, illustrating an example of acombination of two birefringent plates having equal refractive indicesand different thicknesses;

FIG. 14 presents a schematic structure of the optical low-pass filter inan embodiment of the present invention, illustrating an example of acombination of two birefringent plates having different thicknesses andrefractive indices;

FIG. 15 illustrates an example in which the present invention is adoptedin a camera;

FIG. 16 is an enlargement of the area in which the optical filter andthe imaging device are provided in the camera;

FIGS. 17A-17D illustrate structural examples of the conductiveconnection portions through which the electrical charge stored at theoptical filter is released;

FIG. 18 illustrates an essential portion of another example in which thepresent invention is adopted in a camera;

FIG. 19 illustrates an essential portion of yet another example in whichthe present invention is adopted in a camera;

FIG. 20 illustrates an example in which the present invention is adoptedin a camera having a relay lens system; and

FIG. 21 illustrates an example in which the present invention is adoptedin an image reading device (image scanner).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Advantage of Space Saving Achieved by Reducing the Total Thickness ofthe Optical Filter

FIG. 1 is a perspective illustrating an example of the optical filteraccording to the present invention. The optical filter 1 in FIG. 1comprises four main components, i.e., four optical elements each formedin a plate shape, i.e., a first birefringent plate 1 a, an IR blockingfilter 1 b, a phase plate 1 c and a second birefringent plate 1 d. Thefirst birefringent plate 1 a and the second birefringent plate 1 d arepositioned by ensuring that the direction in which an image shift occursas a result of the birefringence achieved by the first birefringentplate 1 a and the direction in which the image shift occurs as a resultof the birefringence achieved by the second birefringent plate 1 d areoffset from each other by 90°. The IR blocking filter 1 b for cuttingoff infrared light and the phase plate 1 c for converting linearlypolarized light to circularly polarized light are provided between thetwo birefringent plates 1 a and 1 d. The phase plate 1 c may beconstituted of, for instance, a quarter-wave plate. It is necessary thatthe phase plate 1 c be provided between the two birefringent plates 1 aand 1 d in this manner, and in addition, since the IR blocking filter 1b turns milky when it comes in contact with air, it is normally enclosedby substrates to ensure that its surface does not come in contact withair. It is to be noted that while the IR blocking filter 1 b may beconstituted by vapor-depositing a multilayer film having an IR blockingeffect on the surface of a glass substrate, a multilayer film similar tothat mentioned above may be provided at a surface of the firstbirefringent plate 1 a or the second birefringent plate 1 d, instead. Inthis case, it is desirable that a protective layer be provided to ensurethat the multilayer film does not come in contact with air. By providinga multilayer film at the surface of the first birefringent plate 1 a orthe second birefringent plate 1 d in this manner, the total thickness ofthe optical filter 1 can be reduced.

Next, in reference to FIG. 2, the functions achieved by the opticalfilter 1 structured as illustrated in FIG. 1 are explained. It is to benoted that in FIG. 2, the IR blocking filter 1 b is not shown to achievesimplicity in the explanation and that the first birefringent plate 1 aand the phase plate 1 c, and the phase plate 1 c and the secondbirefringent plate 1 d are shown separate from each other.

When a light ray L that has been transmitted through a taking lens 20enters the first birefringent plate 1 a, it is separated into linearlight (an ordinary ray L10) that vibrates in a direction perpendicularto the direction in which the light flux advances and linear light(extraordinary ray L20) that vibrates perpendicular to the ordinary rayL10). Since the first birefringent plate 1 a has different refractiveindices for the ordinary ray L10 and the extraordinary ray L20, thephotographic light ray L that becomes the ordinary ray L10 and theextraordinary ray L20 after entering the birefringent plate 1 a travelthrough two separate optical paths, achieves a double image. In thisstructure, with the direction in which the extraordinary ray L20 isshifted relative to the ordinary ray L10 (the horizontal direction inthe figure) referred to as a first direction, the first birefringentplate 1 a can be considered to be an optical element that spatiallydivides input light along the first direction extending perpendicular tothe direction in which the input light flux advances to achieve twoseparate light fluxes. These two light fluxes, i.e., the ordinary rayL10 and the extraordinary ray L20 are linear light fluxes achieving alight intensity ratio of 1:1 and having polarization planes intersectingeach other orthogonally, since the light ray L is natural light.

Next, the ordinary ray L10 and the extraordinary ray L20 enter the phaseplate 1 c. The phase plate 1 c, which is provided to convert linearlight to circular light, converts the ordinary ray L10 and theextraordinary ray L20 to circular ray L10′ and circular ray L20′respectively with their phases offset from each other by 90°. Since abirefringent plate has an effect on circular light that is similar toits effect on natural light under normal circumstances, the circular rayL10′ and the circular ray L20′ that have entered the second birefringentplate 1 d are respectively divided into an ordinary ray L11 and anextraordinary ray L12 having intensities equal to each other, and intoan ordinary ray L21 and an extraordinary ray L22 having intensitiesequal to each other. The direction in which the extraordinary ray L12 isshifted relative to the ordinary ray L11 and the direction in which theextraordinary ray L22 is shifted relative to the ordinary ray L21 bothconstitute a second direction extending perpendicular to the firstdirection discussed earlier (the vertical direction in the figure).

Thus, the photographic light ray L that is originally a single lightflux is first separated into the ordinary ray L10 and the extraordinaryray L20 at the first birefringent plate 1 a, and then after they areconverted to circular light fluxes at the phase plate 1 c by changingthe vibrational planes of the light fluxes, they are separated into fourlight fluxes, i.e., the ordinary rays L11 and L21 and the extraordinaryrays L12 and L22 at the second birefringent plate 1 d. As a result, aquadruple image is formed on an imaging plane 15 a of an imaging device15. Since the first and second birefringent plate 1 a and 1 d arecombined by assuring that the directions in which images are shifted asa result of the birefringence achieved by the two birefringent platesare offset from each other by 90′ as explained earlier, the quadrupleimage on the imaging plane 15 a constitutes a near square shape with theindividual points achieving equal intensity. When the distance betweenthe individual points, which corresponds to the length of one side ofthe square shape, is referred to as a separating distance d, theseparating distance d is calculated through the following equation (26)d=t(ne ² −no ²)/(2ne×no)  (26)witht: birefringent plate thicknessne: extraordinary ray refractive indexno: ordinary ray refractive index

The optical filter 1 according to the present invention is constitutedof the first birefringent plate 1 a, the IR blocking filter 1 b, thephase plate 1 c and the second birefringent plate 1 d. While the phaseplate 1 c is provided between the first birefringent plate 1 a and thesecond birefringent plate 1 d in the structure, the position of the IRblocking filter 16 may be set freely. In other words, the IR blockingfilter 1 b may be provided between the first birefringent plate 1 a andthe phase plate 1 c, as illustrated in FIG. 1, or it may be providedbetween the phase plate 1 c and the second birefringent plate 1 d.Alternatively, the IR blocking filter 1 b may be provided between ataking lens 20 and the first birefringent plate 1 a, or between thesecond birefringent plate 1 d and the imaging plane 15 a.

Next, the materials that may be employed to constitute the firstbirefringent plate 1 a and the second birefringent plate 1 dconstituting the optical filter 1 according to the present invention areexplained. Apart from quartz, lithium niobate (LiNbO₃) is a substanceknown as having a birefringent effect. While lithium niobate is employedto constitute a surface acoustic wave filter in communication devices bytaking advantage of its property whereby it becomes distorted when avoltage is applied and is also employed to constitute a light guide forlaser light by taking advantage of its properties of having a highrefractive index and being transparent, there are almost no examples inwhich its birefringent effect is utilized. However, LiNbO₃, whichachieves a refractive index ne=2.2238 for extraordinary ray of lighthaving a wavelength of 550 nm and a refractive index no=2.3132 forordinary ray at a temperature of 25° C. with a larger difference betweenthe extraordinary ray refractive index and the ordinary ray refractiveindex compared to that of quartz will realize a larger separatingdistance d compared to that of quartz at the same thickness, when itsproperty of causing birefringence is utilized in an optical filter. Forinstance, in order to achieve a separating distance d of 12 μm, “t” iscalculated to be 0.3 mm using equation (26) and the values of therefractive index ne and the refractive index no given above. Thisamounts to only 15% of 2.04 mm required when quartz is used toconstitute the birefringent plates.

FIG. 3 illustrates an example in which the optical filter 1 having itsbirefringent plates constituted of LiNbO₃ according to the presentinvention is mounted in a single lens reflex type digital still camera(DSC). It is to be noted that the figure shows a longitudinal sectionalview illustrating the structure of the DSC.

In the DSC in FIG. 3, a mount 11 for mounting an interchangeable takinglens 20 (see FIG. 3) is provided at its front portion (on the left sidein the figure). However, FIG. 3 illustrates a state in which the takinglens is removed. Subject light L that has been transmitted through thetaking lens 20 is separated into transmitted ray L1 for autofocus (AF)and reflected ray L2 for viewfinder monitoring by a semitransparentquick return mirror 12. When the quick return mirror 12 is lowered,i.e., in a viewfinder monitoring state, the transmitted ray L1 isreflected downward in FIG. 3 by a sub mirror 13 provided as part of thequick return mirror 12 to enter a TTL focus detection device 14 providedat the bottom of the mirror box. The TTL focus detection device receivesthe impinging light through the taking lens, and detects the focal pointof the taking lens. The reflected ray L2, on the other hand, forms animage of the subject on a focal plane 21 a of a viewing screen 21provided at a position that is conjugate with the position of the planeof the imaging device 15 (to be detailed later) and this image isenlarged by an ocular 23 via a penta-prism 22 to be monitored.

When a release button (not shown) is pressed, the quick return mirror 12is caused to swing upward around its pivot portion 12 a together withthe sub mirror 13, to recede to the position indicated by the 2-pointchain line 12′ in the figure. This allows the subject light L that hasbeen transmitted through the taking lens 20 to travel toward the imagingplane 15 a (to be detailed later).

An imaging device package 16 is provided at the rear portion of the DSC(on the right side in the figure). The imaging device package 16 isprovided with the imaging device 15 and a seal glass 2 that covers thefront of the imaging plane 15 a of the imaging device 15. The opticalfilter 1 according to the present invention is provided in closeproximity to the front surface of the seal glass 2. The imaging devicepackage 16 is held by a bracket 17. The bracket 17 is secured to thecamera main body 19 with screws 18. The bracket 17 and the surface ofthe camera main body 19 at which the bracket 17 is mounted are machinedwith a high degree of accuracy so that the imaging plane 15 a of theimaging device package 16 is positioned with a high degree of opticalaccuracy.

A shutter unit 3 is provided between the optical filter 1 and the quickreturn mirror 12 to block light during an imaging signal read operation(signal brigade operation) at the imaging device 15. FIG. 3 illustratesa state during a signal read following exposure, with a light blockingscreen 3 a closed. The shutter unit 3 is formed in such a manner that itopens its light blocking screen 3 a at a start of imaging deviceexposure to allow the subject light L to reach the imaging plane 15 a.

To explain the thickness of the optical filter 1 by referring to FIG. 1again, the total of the thicknesses of the four plate-like opticalelements 1 a, 1 b, 1 c and 1 d constituting the optical filter 1 undernormal circumstances will be 1.6-1.9 mm since the thickness of both thefirst and second birefringent plates 1 a and 1 d constituted of LiNbO₃is 0.3 mm, the thickness of the phase plate 1 c constituted of quartz orthe like is approximately 0.5 mm and the thickness of the IR blockingfilter 1 b is approximately 0.5-0.8 mm with the pixel pitch at theimaging device 15 set at 12 μm, for instance. Even when the totalthickness is at its largest at 1.9 mm, it only amounts to 35% of thetotal thickness 5.38 mm of an optical filter having its first and secondbirefringent plates 1 a and 1 b constituted of quartz, to achieve agreat advantage in space saving. In particular, since only a limiteddegree of freedom in design is allowed in regard to the position of theimaging device 15, whose imaging plane 15 a must be provided at theimage forming plane of the taking lens 20 and, as a result, the spaceavailable for providing the shutter unit 3 and the optical filter 1becomes limited, the advantage of space saving thus achieved incomparison with the space required when the first birefringent plate 1 aand the second birefringent plate 1 d are constituted of quartz issignificant. Most single lens reflex cameras in recent years areprovided with an autofocus adjustment mechanism, with the sub mirror 13provided at the rear of the quick return mirror 12. Thus, there is nolarge space between the rear end of the sub mirror 13 and the shutterunit 3 or between the rear surface of the shutter unit 3 and the lensimage forming plane and, as a result, it is extremely difficult to mountan optical filter having a large thickness of 5.38 mm in a conventionalsilver halide AF camera structure.

However, the optical filter 1 according to the present inventiondescribed above, which saves space, can be mounted in such a structure.Thus, while basically still utilizing the conventional silver halidetype single lens reflex camera structure, a DSC employing an imagingdevice having a large image plane with a pixel pitch exceeding 10 μm canbe realized.

It is to be noted that while LiNbO₃ constituting the first and secondbirefringent plates 1 a and 1 d in the present invention has a cleavingproperty, this shortcoming can be sufficiently compensated by bondingthem together with the phase plate 1 c and the like to achieve anintegrated unit.

In the optical filter 1 structured as described above, an opticalrotatory plate for rotating the plane of polarization of light by 45°may be employed in place of the phase plate 1 c. Examples in which anoptical rotatory plate is used in an optical filter in the prior artprovided with birefringent plates constituted of quartz include thatdisclosed in Japanese Examined Patent Publication No. 1994-20316mentioned earlier. According to the present invention, advantagessimilar to those achieved when the phase plate 1 c explained earlier isemployed can be realized by adopting a structure in which such anoptical rotatory plate is used in combination with the first and secondbirefringent plates 1 a and 1 d constituted of LiNbO₃. This allows for agreater degree of freedom in regard to the structural features otherthan the first and second birefringent plates 1 a and 1 d. Furthermore,it goes without saying that an optical element other than the phaseplate 1 c or the optical rotatory plate may be employed as long as itprovides an effect equivalent to that achieved by the phase plate 1 cand the optical rotatory plate.

FIG. 4 presents a graph illustrating the relationship between the pixelpitch at the imaging device 15 and the thickness of the optical filter 1achieved in a structure having its first and second birefringent platesconstituted of quartz and that achieved in a structure having its firstand second birefringent plates constituted of LiNbO₃. With thethicknesses of the members other than the birefringent plates 1 a and 1d, i.e., the thickness of the IR blocking filter 1 b and the thicknessof the phase plate 1 c set at 0.6 mm and 0.5 mm respectively and thethickness of the birefringent plates 1 a and 1 d which is calculatedthrough equation (26) indicated as t, the thickness “t” of the entireoptical filter is 1.1+340p (p: pixel pitch at the imaging device) usingquartz to constitute the birefringent plates whereas the thickness “t”of the entire optical filter using LiNbO₃ to constitute the birefringentplates is 1.1+50.7p. As the pixel pitch “p” increases, the differencebetween their thicknesses (see Δ₁ and Δ₂ in FIG. 4) clearly becomeslarger. For instance, Δ₁ is 1.16 mm with “p”at 4 μm, Δ₂ is 4.63 mm with“p” at 16 μm and, thus FIG. 4 demonstrates that the larger the pixelpitch, the larger the space saving advantage achieved by constitutingthe birefringent plates 1 a and 1 d with LiNbO₃.

It is to be noted that since the refractive index of LiNbO₃ greatlydiffers from those of quartz and the BK7-equivalent glass constitutingthe IR blocking filter, internal reflection tends to occur more readilyat its boundary surfaces compared to a structure achieved by pastingtogether crystal plates. Since this internal reflection can be preventedby applying an anti-reflection coating to the boundary surface, it isdesirable that an anti-reflection coating be applied at the boundarysurfaces where the plates are pasted together, as well as at the frontsurface as in a regular optical filter.

While the explanation has been given thus far on an example in which theoptical filter 1 is provided to the rear of the shutter unit 3, now anexample in which the position of the optical filter 1 is changed, isexplained below in reference to FIG. 5. It is to be noted that in FIG.5, the same reference numbers are assigned to members and the like withthe same structural features and functions as those illustrated in FIG.3 to preclude the necessity of a repeated explanation thereof.

As illustrated in FIG. 5, a thin optical filter 1 employing LiNbO₃ maybe provided at the front surface of the quick return mirror 12 in asingle lens reflex type DSC. In this case, while the optical filter 1 isstill positioned within the optical path (the photographic light flux L)of the taking lens achieving a similar optical effect in a photographingstate with the quick return mirror raised (indicated by the 2-pointchain line in the figure), the optical filter 1 is present in both theviewfinder monitoring optical path and the AF detection optical path ina viewfinder monitoring state with the quick return mirror 12 lowered(indicated by the solid line and the figure). This is, strictlyspeaking, not desirable since the effect of the image separation at theoptical filter 1 affects the viewfinder image and the AF detectionaccuracy. However, even when there is hardly any gap between the shutterunit 3 and the imaging device 15 and the optical filter cannot bepositioned between the shutter unit 3 and the imaging device 15 evenwith the space saving effect achieved through the use of LiNbO₃, theoptical filter can be mounted between a locus 12 s at the front end ofthe quick return mirror 12 during its operation and a trailing end LB ofthe taking lens by reducing the radius of the locus 12 s at the frontend of the quick return mirror 12 compared to that in a single lensreflex type camera using the 135-type photographic film. If the size ofthe imaging device is smaller (e.g., the APS size) than the image planesize of the 135-type photographic film, the size of the quick returnmirror 12 can be also reduced correspondingly compared to the size ofthe quick return mirror in a camera that employs the 135-typephotographic film. Thus, the space for accommodating the thin opticalfilter 1 constituted by using LiNbO₃ is assured by the reduced radius ofthe locus 12 s at the front end of the quick return mirror 12.

While the explanation has been given thus far on an example in whichboth the first and second birefringent plates 1 a and 1 d in the opticalfilter 1 are constituted of LiNbO₃, a corresponding degree of spacesaving effect can be achieved by forming one of the two birefringentplates 1 a and 1 b with LiNbO₃ and forming the other birefringent platewith, for instance, quartz. In other words, the technical scope of thepresent invention includes a structure achieved by constituting only oneof the two birefringent plates 1 a and 1 d with LiNbO₃, as well as astructure which is achieved by constituting both the birefringent plates1 a and 1 d with LiNbO₃.

Improvement in the Optical Performance Achieved by Reducing the TotalThickness of the Optical Filter

As explained above, by constituting at least either the first or secondbirefringent plate 1 a or 1 d in the optical filter 1 with LiNbO₃, thetotal thickness of the optical filter 1 can be reduced. In addition tothe advantage explained earlier, the optical filter 1 according to thepresent invention achieves an advantage of reducing the focusmisalignment occurring in the direction of the optical axis between thecentral area of the image plane and the periphery of the image plane asexplained below.

FIG. 6 schematically illustrates the structure in which the opticalfilter 1 is provided between the taking lens 20 and the imaging device15 inside the single lens reflex type DSC illustrated in FIG. 3 or FIG.5. It is to be noted that the illustration of the IR blocking filter 1 bis omitted in FIG. 6. In the optical filter 1 in FIG. 6, the thicknessof the first birefringent plate 1 a and the thickness of the secondbirefringent plate 1 d are equal to each other at t1. The thickness ofthe phase plate 1 c is t2. In addition, the refractive index of thefirst birefringent plate 1 a and the second birefringent plate 1 d isn1, whereas the refractive index of the phase plate is n2. A CCD, a MOStype image sensor or the like is employed to constitute the imagingdevice 15.

Now, a general principle is discussed in regard to the total thicknessof an optical filter in reference to a comparison of an optical filterused in combination with an imaging device having a ⅔” image area sizeand approximately 1.3 million pixels and an optical filter used incombination with an imaging device having a 15.5 mm×22.8 mm image areasize and approximately 2 million pixels. Then, optical problemsoccurring as a result of an increase in the thickness of the opticalfilter are explained.

At an imaging device having image plane size of approximately ⅔″ andapproximately 1.3 million pixels, the pixel pitch will be approximately6.6 μm. The thickness “t” required to achieve a separating distance forthe image corresponding to this level of pixel pitch by using quartz,the most common material, to constitute the birefringent plates iscalculated as follows. Quartz has the following refractive indices forlight having a wavelength of 589 nm.

ne=1.55336

no=1.54425

The thickness “t” for each birefringent plate is calculated to beapproximately 1.12 mm by performing the reverse calculation usingequation (26) with d set at 6.6 μm. As already explained, since thethickness of the phase plate needs to be approximately 0.5 mm regardlessof what the separating distance is, the thickness of the optical filterconstituted by pasting together the three plates in this case will beapproximately 2.74 mm.

Now, an imaging device having an image area of approximately 15.5mm×22.8 mm and having approximately 2 million pixels, which iscomparable to the C-type in the IX240 system will have a pixel pitch ofapproximately 13.2 μm. The thickness “t” required for achieving aseparating distance for the image corresponding to the pixel pitch of13.2 μm when constituting the birefringent plates with quartz, as in theexample featuring the ⅔″ imaging device, is calculated to be “t”=2.25 mmby performing reverse calculation using equation (26) with d set at 13.2μm. The total thickness of the optical filter is calculated to beapproximately 5 mm by adding the thickness of the two birefringentplates and the thickness 0.5 mm of the phase plate, which shows anapproximately 82% increase over the thickness of the optical filterhaving a pixel pitch of 6.6 μm. When the thickness 0.5 mm of the IRblocking filter is added, the optical filter supporting the pixel pitchof 13.2 μm will have a large thickness of 5.5 mm adding together thethicknesses of the four elements including that of the IR blockingfilter.

The reason for focus misalignment occurring in the direction of theoptical axis between the central area of the image plane and theperiphery of the image plane as the thickness of the optical filterincreases in this manner is explained in reference to FIG. 7.

For the purpose of simplifying the explanation, it is assumed that anoptical filter OF having a total thickness “t” which is provided betweenthe taking lens 20 and the imaging device 15 has a uniform refractiveindex n. By providing the optical filter OF between the taking lens 20and the imaging device 15, the air equivalent optical path lengthbetween the lens 20 and the imaging plane 15 a of the imaging device 15changes as expressed in the following equation (27) relative to astructure with no optical filter OF present. $\begin{matrix}{{\Delta\quad 1} = {t \times \left( {1 - \frac{1}{n}} \right)}} & (27)\end{matrix}$

The change quantity Δ₁ in the air equivalent optical path lengthexpressed through equation (27) relates to a beam of light advancing onthe optical axis Ax of the taking lens 20 and is achieved only when thelight flux enters the optical filter OF perpendicularly. The angle ofincidence ø of the light flux entering the filter OF after passingthrough the center “P” of the exit pupil L of the lens 20 is at itslargest when the light flux SR enters an off-axis corner 15 f of theimaging plane 15 a of the imaging device 15. With θ₁ representing therefractive angle of the light flux SR after it enters the optical filterOF and L1 representing the optical path length of the light flux SRwithin the optical filter OF, the change quantity Δ₃ of the airequivalent optical path length along the direction in which the lightflux SR advances is calculated through the following equation (28).Then, based upon equation (28) the change quantity Δ₂ of the airequivalent optical path length in the direction of the optical axis iscalculated through the following equation (29). $\begin{matrix}\begin{matrix}{{\Delta\quad 3} = {{\frac{t}{\cos\quad\phi} - \frac{L1}{n}} = {\frac{t}{\cos\quad\phi} - \frac{t}{n \times \cos\quad\theta\quad 1}}}} \\{= {t \times \left( {\frac{1}{\cos\quad\phi} - \frac{1}{n \times \cos\quad\theta\quad 1}} \right)}}\end{matrix} & (28) \\{{\Delta\quad 2} = {{\Delta\quad 3 \times \cos\quad\phi} = {t \times \left( {1 - \frac{\cos\quad\phi}{n \times \cos\quad\theta\quad 1}} \right)}}} & (29)\end{matrix}$

Equation (27) and equation (29) indicate that a difference Δ₂-Δ₁ asexpressed through the following equation (30) is formed in the changequantity of the air equivalent optical path length between the lightflux reaching the center 15 c of the imaging plane 15 a of the imagingdevice 15 and the light flux reaching the diagonal corner 15 f. Thisdifference causes a focus misalignment occurring between the image planecenter and the image plane periphery, resulting in the image formingplane of the taking lens 20 becoming non-planar. As a result, as thedifference Δ₂-Δ_(l) increases, the quality of image at the periphery ofthe image plane deteriorates. $\begin{matrix}{{{\Delta\quad 2} - {\Delta 1}} = {\frac{t}{n} \times \left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}} \right)}} & (30)\end{matrix}$

As equation (30) clearly indicates, since it can be assumed that θ₁≈φeven in the case of a light flux entering a diagonal corner 35 a of theimage plane as long as the angle of incidence of the light flux at theoptical filter OF is not excessively large, the difference Δ₂-Δ₁ can beconsidered to be 0.

The following two conditions must be satisfied to ensure that the angleof incidence at the optical filter OF of a light flux entering adiagonal corner of an image plane does not become excessively large.Specifically, the first condition is that the distance (PO) between theexit pupil L of the taking lens 20 and the imaging plane 15 a is large.The second condition is that the image area of the imaging device 15 besmall, i.e., that the area of the imaging plane 15 a be small with asmall distance “A” achieved between the image plane center 15 c and thediagonal corner 15 f of the image plane.

Since interchangeable lenses used in a camera employing the 135-typephotographic film can be often directly utilized in a single lens reflextype DSC, the lenses utilized in such a DSC may have a relatively shortPO of approximately 50 mm. The difference Δ₂-Δ₁ explained abovemanifesting when the imaging size of the imaging device is, forinstance, 24 mm×16 mm (aspect ratio; 3:2) and the pixel pitch is 13.2 mmand a taking lens 20 with a PO of 50 mm is mounted, is now calculated.

When the thickness of the optical filter OF is assumed to be 5 mm (thepixel pitch at 3.2 μm, includes the thickness 0.5 mm of the phase plate)and “n”=1.54 (quartz), the image height at the image plane is calculatedto be 14.4 mm with the angle of incidence φ at the optical filter OF at16.1°. Based upon the law of refraction, θ₁ is calculated to be 10.4°,and based upon equation (30), the focus misalignment quantity Δ₂-Δ₁ atthe image plane center 15 c and the diagonal corner 15 f of the imageplane is calculated to be approximately 75 μm.

When the results of the calculation performed above are compared withthose achieved by an optical filter employed in combination with animaging device in the ⅔″ size (the image height 5.6 mm at the corner,the pixel pitch at 6.6 μm) described earlier and a lens with PO set at100 mm, θ₁ is calculated to be 2.1° based upon the angle of incidenceφ=3.2°. Since the pixel pitch of the ⅔″ size imaging device is 6.6 μmand the thickness of the optical filter OF (n=1.54) is calculated to beapproximately 2.7 mm (includes the thickness of the phase plate), thefocus misalignment quantity Δ₂-Δ₁ at the diagonal corner of the imageplane in this case is calculated to be approximately 1.6 μm, which isonly 2% of 75 μm.

Now, when discussing the depth of focus achieved by imaging with animaging device, the setting of the allowable diameter of the circle ofconfusion which constitutes a premise for the discussion, i.e., thesetting of the allowable circle of confusion diameter, is an issue to beaddressed. There is a theory that the allowable circle of confusiondiameter should be set to approximately 33 μm (= 1/30 mm) with the135-type (35 mm full size) photographic film. At the same time, whilethere are various theories in regard to how the allowable circle ofconfusion diameter should be set when imaging is performed with animaging device, they all fall within a range of 1 time to approximately3 times the pixel pitch of the imaging device. In this discussion weshall assume the allowable circle of confusion diameter is twice thepixel pitch.

The depth of focus is calculated as the product of aperture valuesetting at the taking lens and the allowable circle of confusiondiameter. Thus, the depth of focus achieved when the aperture valuesetting at the taking lens is F2.8 and the pixel pitch is 13.2 μm iscalculated to be 2.8×13.2 μm×2=74 μm. In addition, the depth of focusachieved when the aperture value setting at the taking lens is F 2.8 andthe pixel pitch is 6.6 μm is calculated to be 2.8×6.6 μm×2=36 μm. Whilea focus misalignment of 1.6 μm relative to the depth of focus of 36 μmdoes not present any problem whatsoever, a focus misalignment of 75 μmrelative to the depth of focus of 74 μm poses a serious problem. Even ifthe focal point matches perfectly without any error at the center of theimage plane, the focus misalignment attributable to the thickness of theoptical filter OF already exceeds the depth of focus at the corners ofthe image plane, and if we also take into consideration error factors inregard to the focal point matching at the center of the image plane(positioning adjustment accuracy at the imaging plane, lens focusingerror and the like), there will be no room for allowance for errorfactors left for the image at the corners of the image plane.

FIG. 8 presents a graph illustrating the relationship between the angleof incidence of a light flux at an optical filter and the focusmisalignment quantity discussed above with the thickness of the opticalfilter is as a parameter. In the graph in FIG. 8, a curve 1 representsthe change in the focus misalignment quantity observed when a relativelythick optical filter employing quartz birefringent plates is used tosupport an imaging device having a pixel pitch of 13.2 μm, whereas curve2 represents change in the focus misalignment quantity observed when arelatively thin optical filter employing quartz birefringent plates isused to support an imaging device having a pixel pitch of 6.6 μm. Point“T” indicates the results achieved when PO=50 mm, the pixel pitch is13.2 μm and the image height at the corner is 14.4 mm, whereas point “S”indicates the results achieved when PO=100 mm, the pixel pitch is 6.6 μmand the image height at the corner is 5.6 mm.

When the imaging device having a pixel pitch of 6.6 μm is enlarged toachieve the dimensions 24 mm×16 mm, the focus misalignment quantity atthe corners of the image plane increase diagonally upward to the rightalong the curve 2 from the point “S” to reach the value indicated bypoint “S1” with the angle of incidence at 16.1°. However, in a largerimaging device, the pixel pitch is also set larger in consideration ofthe comparative merits achieved by an increase in the number of pixelsrelative to the production yield and also in order to improve thesensitivity. As a result, the thickness of the optical filterconstituted by using quartz must be increased in correspondence.

Due to this increase in the thickness of the optical filter, the focusmisalignment quantity further increases upward from the point “S1” onthe curve 2 until the focus misalignment quantity is at the point “T” onthe curve 1 in the case of the imaging device having a pixel pitch of13.2 μm. The focus misalignment quantity at the corners of the imageplane increases markedly in an imaging device having a large image areain this manner, since the increase in the angle of incidence at thefilter of light entering the corners of the image plane at the filterand the increase in the pixel pitch resulting in an increase in thefilter thickness are factors which together introduce a greater effectthan any one of them alone.

FIG. 9 presents a graph similar to that presented in FIG. 8. In FIG. 9,a curve representing the change in the focus misalignment quantityobserved when an optical filter employing quartz birefringent plates isused to support an imaging device having a pixel pitch of 13.2 μm and acurve representing the change in focus misalignment quantity observedwhen an optical filter employing quartz birefringent plates is used tosupport an imaging device having a pixel pitch of 9 μm are presented.

Now, let us assume that the allowable value for the focus misalignmentdescribed above is ⅓ of the depth of focus. When the aperture valuesetting at the taking lens is at F 2.8, the depth of focus is calculatedas 2.8×pixel pitch×2. Using the pixel pitch, the allowable value for thefocus misalignment is expressed as 2.8× pixel pitch×2.3=1.9× pixelpitch. FIG. 9 indicates that the 1.9× pixel pitch focus misalignment(25.1 μm with a pixel pitch of 13.2 μm and 17.1 μm with a pixel pitch of9 μm) occurs when the angle of incidence is approximately 9.5°. It isindicated that since TAN 9°=0.158, a filter that takes intoconsideration the focus misalignment occurring at the corners of theimage plane when there is a possible combination of “PO” and the imageheight at the corner “A” that results in TAN φ exceeding TANφ=A/PO≧=0.15 must be achieved.

Now, an explanation is given in regard to how the focus misalignmentdescribed above is reduced by employing the optical filter 1 in theembodiment of the present invention, again in reference to FIG. 6. Asillustrated in FIG. 6, the light flux SR travels through the center ofthe exit pupil L of the taking lens 20 to enter the corner 15 f alongthe diagonal of the imaging plane 15 a, and 01 represents the refractiveangle of the light flux SR after it enters the first birefringent plate1 a (since the first birefringent plate 1 a and the second birefringentplate 1 d have the same refractive index, the refractive angle of thelight flux SR after it enters the second birefringent plate 1 d, too, isreferred to as θ₁). Likewise, the refractive angle of the light flux SRafter it enters the phase plate 1 c is indicated as θ₂. In addition, n1represents the refractive index of the first birefringent plate 1 a andthe second birefringent plate 1 d, and n2 represents the refractiveindex of the phase plate 1 c.

Based upon equation (30), the individual focus misalignment quantitiesat the corners of the image plain occurring as a result of the lightbeing transmitted through the first birefringent plate 1 a, the phaseplate 1 c and the second birefringent plate 1 d, are calculated, withthe focus misalignment quantities corresponding to the birefringentplates 1 a and 1 d calculated by using the following equation (31) andthe focus misalignment quantity corresponding to the phase plate 1 ccalculated by using the following equation (32) respectively. The focusmisalignment quantity Aa occurring at the corners of the image planeattributable to the entire optical filter 1 is the total of theindividual focus misalignment quantities, which may be calculatedthrough the following equation (33). $\begin{matrix}{{\begin{matrix}{{focus}\quad{misalignment}\quad{quantity}} \\\left( {{birefringent}\quad{plate}} \right)\end{matrix}\quad\Delta\quad r} = {\frac{t1}{n1} \times \left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}} \right)}} & (31) \\{{\begin{matrix}{{focus}\quad{misalignment}\quad{quantity}} \\\left( {{phase}\quad{plate}} \right)\end{matrix}\quad\Delta\quad p} = {\frac{t2}{n2} \times \left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right)}} & (32) \\{{\Delta\quad a} = {{2 \times \Delta\quad r} + {\Delta\quad p}}} & (33)\end{matrix}$

The concept of the allowance for the focus misalignment quantity at thecorners of the image plane is explained again. At the focus position atthe center of the image plane (=on the lens optical axis), there isalmost always a focusing error (the range finding error and the lensstop position accuracy error attributable to autofocus, or focusingerror in manual range finding), and it is also difficult to achieve zeroerror for the mechanical accuracy with respect to the image planeposition of the camera itself. Thus, these errors must be ultimatelycovered with the depth of focus at the image plane (=product of theaperture value setting at the taking lens and the allowable circle ofconfusion diameter). As a result, the allowable value for the focusmisalignment quantity cannot be set equal to the depth of focus, and itmust be ensured that the allowable value for the focus misalignmentquantity must be set equal to or less than a factor K (K<1) of the depthof focus. By taking into consideration the deviation factor of thefocusing accuracy described above at the center of the image plane, avalue that is approximately ¼-⅓ (0.25≦K≦0.35) of the depth of focus maybe regarded as reasonable.

At the same time, since there are various theories in regard to thelength of the allowable circle of confusion diameter relative to thepixel pitch at the imaging plane of an imaging device, all of which fallwithin the range of approximately 1˜3 times the pixel pitch, asexplained earlier, the allowable circle of confusion diameter isexpressed as B×d (1≦B≦3,d: pixel pitch)

With Fno representing the aperture value setting at the taking lens 20,the relationship described above is numerically expressed throughequation (34). $\begin{matrix}{{{2 \times \frac{t1}{n1} \times \left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}} \right)} + {\frac{t2}{n2} \times \left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right)}} \leqq {K \times B \times d \times {{Fno}\left( {{0.25 \leqq K \leqq 0.35},{1 \leqq B \leqq 3}} \right)}}} & (34)\end{matrix}$

By fixing the distance from the exit pupil L of the taking lens 20 tothe imaging plane 15 a and the image area size of the imaging device 15at constant values and by constituting the phase plate with a specificmaterial to a specific thickness (e.g., using quartz which is a commonmaterial for this application, to achieve a thickness of approximately0.5 mm), constants, i.e., n2=1.54, θ₂=10.4° and t2=0.5, are achieved. Inaddition, since φ, too, achieves a constant value, the second term inthe left side member of equation (34) becomes a constant. Thus, θ₁ canbe expressed as a function of n1 so that equation (34) is re-expressedwith the following equation (35). $\begin{matrix}{{{t1} \leqq {C \times \frac{n1}{Y({n1})}}}{with}} & (35) \\{{Y({n1})} = {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}}} & (36) \\{C = {{\frac{1}{2} \times \left\{ {{K \times B \times d \times {Fno}} - {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right) \times \frac{t2}{n2}}} \right\}} = {{const}.}}} & (37) \\{{\sin\quad\theta\quad 1} = \frac{\sin\quad\phi}{n1}} & (38) \\{{\theta\quad 1} = {\sin^{- 1}\left( \frac{\sin\quad\phi}{n1} \right)}} & (39) \\{{\theta\quad 2} = {\sin^{- 1}\left( \frac{{n1} \times \sin\quad\phi}{n2} \right)}} & (40)\end{matrix}$

Now, let us consider a case in which the image height is 14.4 mm (adiagonal of 24 mm×16 mm), the air equivalent optical path length fromthe image plane to the exit pupil of the lens is 50 mm, K=0.3, B=3, d=12μm and Fno=2.8. Y(n1) and C are calculated using the following equations(41) and (42) with the angle of the incidence φ at the filter at 16.1°and θ₂ at 10.4°. $\begin{matrix}{{Y({n1})} = {1 - \frac{0.9608}{\cos\left\{ {\sin^{- 1}\left( {0.2773/{n1}} \right)} \right\}}}} & (41) \\{C = 0.006318} & (42)\end{matrix}$

By rendering in a graph the relationship expressed in equation (35) withthese values incorporated, the horizontal axis representing n1 and thevertical axis representing t1, a curve G in FIG. 10 is achieved. Thecombination of t1 and n1 that satisfies the inequality expressed inequation (35) is present in the range that is lower than the curve G inFIG. 10. By making an appropriate selection for the material toconstitute the first birefringent plate 1 a and the second birefringentplate 1 d to ensure that the combination of the refractive index n1 andthe thickness t1 described above is achieved, the focus misalignmentquantity at the corners of the image plane is set to be equal to orlower than a specific value (the allowable value set by selecting thevalues for B and K). For instance, let us consider a case in whichquartz is used to constitute the first birefringent plate 1 a and thesecond birefringent plate 1 d. t1q representing the thickness achievedby constituting the first birefringent plate 1 a and the secondbirefringent plate 1 d of quartz is calculated to be t1q=2.04 mm byperforming reverse calculation using equation (26) with d=12 μm. Sincethe refractive index n1q of quartz is 1.54, the coordinates (n1q, t1q)in the graph in FIG. 10 correspond to the position of point Q, therebydemonstrating that the focus misalignment quantity at the corners of theimage plane is not set to be equal to or less than the allowable value.

Now, let us consider a case in which lithium niobate (LiNbO₃), which isknown as a material having a birefringent effect comparable to thatachieved by quartz, is used to constitute the first birefringent plate 1a and the second birefringent plate 1 d. The extraordinary rayrefractive index ne and the ordinary ray refractive index no of lithiumniobate are ne=2.2238 and no=2.3132 (=nib) respectively. t1brepresenting the thickness of the birefringent plates 1 a and 1 d withlithium niobate is calculated to be t1b=0.3 mm by incorporating d=12 μmand the value of ne and no above in equation (26).

The position of the coordinates (n1b, t1b) is indicated at point NB inthe graph in FIG. 10. The point NB is located in the range lower thanthe curve G, demonstrating that the focus misalignment easily stayswithin the allowable value.

Next, FIG. 11 presents a graph with K=0.25 and B=1.5 adopting morerigorous criteria for evaluation. By these criteria for evaluation, thepoint NB achieved by using lithium niobate to constitute thebirefringent plates 1 a and 1 d is located in the range above the curveG′, and the focus misalignment quantity is no longer equal to or lessthan the allowable value. It is understood that in this case, a focusmisalignment quantity equal to or less than the allowable value isachieved by using Chilean nitrate (NaNO₃, ne=1.34, no=1.60) indicated bya point NA or rutile (TiO₂, ne=2.9, no=2.61) indicated by a point TI inFIG. 11 to constitute the birefringent plates 1 a and 1 d.

The level of rigor for the evaluation criteria is set depending upon theselected values for K and B, the F-number and the position of the exitpupil of a taking lens that can be mounted, and the required imagedistance over which focusing should be assured. In a camera that doesnot allow lens exchange, the open F-number of the lens is naturallyselected for the F-number, whereas in an interchangeable lens typecamera, the open F-number of the lens achieving the brightest openF-number among lenses that may be mounted is selected for the F-number,under normal circumstances. The same principle applies to the positionof the exit pupil. In addition, K and B are set within the settingranges explained earlier, in reference to the overall target performancelevel and the like of the electronic camera in which the optical filteris mounted.

For instance, if a lens having an open F-number of 1.4, which isincluded among the lineup of interchangeable lenses that can be used isexpected to be used, it is naturally necessary to set the targetperformance corresponding to this open F-number. FIG. 12 presents theresults of the evaluation (NB) obtained by using lithium niobate toconstitute the first birefringent plate 1 a and the second birefringentplate 1 d with K=0.33, B=3 and Fno=1.4.

The explanation has been given thus far on an example in which theoptical filter 1 according to the present invention is employed incombination with an imaging device constituted through a so-calledsquare pixel array whereby the pixels at the imaging device 15 arearrayed in the same array pitch in both the longitudinal and the lateraldirections. However, the imaging device 15 is not necessarily requiredto assume a square pixel array. If the imaging device 15 does not assumea square pixel array, it is necessary to set different separatingdistances for the image effected by the two birefringent plates 1 a and1 d corresponding to the array pitches in the longitudinal and lateraldirections. In such a case, a method whereby the two birefringent plateshaving different thicknesses are constituted of the same material, amethod whereby the two birefringent plates are constituted of materialshaving different refractive indices to achieve the same thickness or amethod whereby the two birefringent plates are formed to have differentthicknesses and different refractive indices from each other, may beadopted.

FIG. 13 illustrates an example in which a single material is used toconstitute the two birefringent plates, formed to have differentthicknesses. In FIG. 13, with t11 and t12 respectively representing thethicknesses of a first birefringent plate 1Aa and a second birefringentplate 1Ad constituting an optical filter 1A provided between the takinglens 20 and an imaging device 15A, a conditional equation correspondingto equation (35) is presented as the following equation (43).$\begin{matrix}{{{{t\quad 11} + {t\quad 12}} \leqq {C\quad 1 \times \frac{n\quad 1}{Y\left( {n\quad 1} \right)}}}{with}} & (43) \\{{Y\left( {n\quad 1} \right)} = {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}}} & (44) \\{{C\quad 1} = {{{K \times B \times d \times {Fno}} - {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right) \times \frac{t\quad 2}{n\quad 2}}} = {{const}.}}} & (45) \\{{\sin\quad\theta\quad 1} = \frac{\sin\quad\phi}{n\quad 1}} & (46) \\{{\theta\quad 1} = {\sin^{- 1}\left( \frac{\sin\quad\phi}{n\quad 1} \right)}} & (47) \\{{\theta\quad 2} = {\sin^{- 1}\left( \frac{n\quad 1 \times \sin\quad\phi}{n\quad 2} \right)}} & (48)\end{matrix}$

When the thicknesses of the two birefringent plates are different, agraph corresponding to that in FIG. 10 is drawn in a similar manner byusing (n1, t11+t12) as a variable. The selected birefringent material isevaluated at a point with the X coordinate value represented by itsrefractive index n1 and the Y coordinate value represented by the total(t11+t12) of the thicknesses t11 and t12 determined by the requiredimage separating distances.

An example constituted of two birefringent plates having differentthicknesses and different refractive indices is illustrated in FIG. 14.With t11 and t12, and n11 and n12 respectively representing thethicknesses and the refractive indices of a first birefringent plate 1Baand a second birefringent plate 1Bd constituting an optical filter 1Bprovided between the taking lens 20 and the imaging device 15A in FIG.14, a conditional equation corresponding to equation (35) is presentedas the following equation (49). $\begin{matrix}{{{{\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 1}} \right) \times \frac{t\quad 11}{n\quad 11}} + {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 3}} \right) \times \frac{t\quad 12}{n\quad 12}}} \leqq {C\quad 2}}{with}} & (49) \\{{C\quad 2} = {{{K \times B \times d \times {Fno}} - {\left( {1 - \frac{\cos\quad\phi}{\cos\quad\theta\quad 2}} \right) \times \frac{t\quad 2}{n\quad 2}}} = {{const}.}}} & (50) \\{{\sin\quad\theta\quad 1} = \frac{\sin\quad\phi}{n\quad 11}} & (51) \\{{\theta\quad 1} = {\sin^{- 1}\left( \frac{\sin\quad\phi}{n\quad 11} \right)}} & (52) \\{{\theta\quad 2} = {\sin^{- 1}\left( \frac{n\quad 11 \times \sin\quad\theta\quad 1}{n\quad 2} \right)}} & (53) \\{{\theta\quad 3} = {\sin^{- 1}\left( \frac{n\quad 2 \times \sin\quad\theta\quad 2}{n\quad 12} \right)}} & (54)\end{matrix}$

While the explanation has been given in reference to the embodimentsabove on an example in which the present invention is adopted in anoptical filter for a DSC, the present invention may be adopted in othertypes of cameras provided with a solid-state imaging device such as avideo camera or in an optical apparatus such as an image scanner or thelike.

While the explanation has been given above with respect to the advantageof reducing the degree of focus misalignment occurring in the directionof the optical axis between the central area of the image plane and theperiphery of the image plane by reducing the thickness of the opticalfilter in reference to an example in which only the optical filter isprovided between the taking lens 20 and the imaging device 15, aninfrared blocking filter may be provided together with the opticalfilter. It is obvious that the focus misalignment attributable to thethickness of the infrared blocking filter must also be taken intoconsideration in this case.

In addition, in the explanation given above, the first birefringentplate 1 a and the second birefringent plate 1 d constituting the opticalfilter 1 utilized in combination with the imaging device 15 having thesquare pixel array illustrated in FIG. 6 have the same thickness t1 andthe same refractive index n1. However, the present invention is notrestricted to this example. Namely, the separating distance achievedwhen an incident light flux is separated into two spatially separatelight fluxes by one birefringent plate is determined by the product ofthe thickness and the refractive index of the birefringent plate andthus, two birefringent plates having different thicknesses andrefractive indices may be adopted in combination in an imaging devicehaving a square pixel array. Furthermore, the relative angle of thedirections in which the light is separated by the first and secondbirefringent plates is not restricted to 90°, and various angles may beset including 30°, 45°, 60° and so forth, depending upon the separationpattern to be achieved.

Prevention of Shadows of Foreign Matter Cast onto Input Image

FIG. 15 schematically illustrates an essential portion of aninterchangeable lens type single lens reflex type DSC in which thepresent invention is adopted (hereafter, an interchangeable lens typesingle lens reflex DSC is simply referred to as a “camera”). A takinglens 20 which can be exchanged to suit particular purposes ofphotographing is mounted at a camera main body 70. Inside a mirror box72 of the camera main body 70, a mirror 12, a shutter unit 3, an opticalfilter 1, an imaging device 15 and the like are provided. A focusingscreen 21 is provided above the mirror box 72.

The mirror 12 is automatically switched between a lowered state i.e.,the state indicated by the solid line in FIG. 15, and a raised state,i.e., the state indicated by the 2-point chain line in FIG. 15, incorrespondence to the operating state of the camera. The shutter unit 3blocks light during an imaging signal read operation at the imagingdevice 15 which is provided, together with the optical filter 1, to therear of the shutter unit 3. Since the optical filter 1 assumes astructure identical to that illustrated in FIG. 1, its explanation isomitted.

FIG. 16 is an enlargement of the area where the optical filter 1 and theimaging device 15 are provided in the camera illustrated in FIG. 15.Transparent electrodes 38A and 38B constituted of a Nesa film or thelike are respectively formed at the entry surface and the exit surfaceof the optical filter 1. A transparent electrode 38C constituted of aNesa film or the like is also formed at the front surface of a sealglass 2 provided at the photosensitive surface of the imaging device 15.These transparent electrodes 38A-38C are connected to a conductive areaof a casing 37 via a conductive connection portion 36 a or 36 b asdetailed later.

The optical filter 1 and the casing 37, the conductive connectionportions between the transparent electrodes 38A and 38B formed at thefront surface of the optical filter 1 and the casing 37 are explained inreference to FIGS. 17A-17D, which illustrate these components in partialenlargements. In FIG. 17A, a through hole 34 a is bored at the opticalfilter 1. A conductive pin 91 is inserted through the through hole 34 a,and the pin 91 and the transparent electrodes 38A and 38B are bonded byusing a conductive adhesive 90 such as silver paste. This achieves anelectrically continuous state for the pin 91 and the transparentelectrodes 38A and 38B. One end of a wire 92 is soldered onto the pin91, with another end of the wire 92 soldered to the conductive area ofthe casing 37. It is to be noted that the wire 92 and the conductivearea of the casing 37 may be connected with each other by attaching alug plate or the like (not shown) to the wire 92 through soldering orcrimping and securing the lug plate or the like to the casing 37 throughscrewing. Thus, the transparent electrodes 38A and 38B are connected tothe conductive area of the casing 37 so that their potentials are setequal to the potential at the conductive area. As described above, aconductive connection portion 36 aA in the example illustrated in FIG.17A is constituted of the conductive adhesive 90, the pin 91 and thewire 92.

The conductive connection portion at which the transparent electrodes38A and 38B are connected with the conductive area of the casing 37 mayassume a structure illustrated in FIG. 17B or FIG. 17C, instead, and thefollowing is an explanation of these structures. In FIG. 17B, conductivemembers 94A and 94B having a spring property are secured at theconductive area of the casing 37 in a state that allows electricalcontinuity. A resilient restoring force imparted by the conductivemembers 94A and 94B presses the transparent electrode 38A in contactagainst the conductive member 94A and the transparent electrode 38B incontact against the conductive member 94B respectively. Thus, thetransparent electrodes 38A and 38B are both connected to the conductivearea of the casing 37 with their potentials set equal to that at theconductive area. In other words, in the example illustrated in FIG. 17B,a conductive connection portion 36 aB is constituted of the conductivemembers 94A and 94B.

In FIG. 17C, the optical filter 1 is held by a frame body 95 having aconductive property, and the transparent electrodes 38A and 38B are incontact with the frame body 95 in a state that allows electricalcontinuity. The frame body 95 is retained at a fixing portion 37 aformed at the conductive area of the casing 37 through tightening ofscrews 96. Thus, the transparent electrodes 38A and 38B are bothconnected to the conductive area of the casing 37 with their potentialsset equal to that at the conductive area. In other words, a conductiveconnection portion 36 aC in the example presented in FIG. 17C isconstituted of the frame body 95, the screws 96 and the fixing portion37 a.

Now, in reference to FIG. 17D, the conductive connection portion 36 b atwhich the transparent electrode 38C formed at the front surface of theseal glass 2 of the imaging device 15 and the conductive area of thecasing 37 are connected is explained. A conductive member 94C having aspring property is secured at the bracket 17 holding the imaging device15, with the conductive member 94C placed in contact with thetransparent electrode 38C. One end of a wire 97 is soldered onto theconductive member 94C, with another end of the wire 97 soldered onto theconductive area of the casing 37. As in the conductive connectionportion 36 aA illustrated in FIG. 17A, the wire 97 and the casing 37 maybe connected by attaching a lug plate (not shown) to the wire 97 throughsoldering or crimping and by retaining the lug plate or the like at thecasing 37 through screwing. As explained above, the conductiveconnection portion 36 b in the example illustrated in FIG. 17D isconstituted of the conductive member 94C and the wire 97.

In the camera structured as described above, the mirror 12 is in alowered state as illustrated in FIG. 15 during a photographingpreparation operation, i.e., when the photographer is engaged in anoperation related to framing (monitoring of a viewfinder image),adjustment of the exposure value, focal point adjustment and the like.Thus, the image of the subject formed by the taking lens 20 is reflectedupward by the mirror 12 so that an image can be formed on the focusingscreen 21. The photographer monitors the subject image formed on thefocusing screen 21 via a viewfinder lens system (not shown).

During a photographing operation, the mirror 12 swings upward andsubsequently, after the shutter unit 3 has been engaged in an open/closeoperation, the mirror 12 is lowered. Through this sequence ofoperations, the light from the subject guided by the taking lens 20 istransmitted through the optical filter 1 to enter the imaging device 15.

In the camera described above, the transparent electrodes 38A and 38Bformed at the two surfaces of the optical filter 1 and the transparentelectrode 38C formed at the front surface of the seal glass 2 at theimaging device 15 are all connected to the conductive area of the casing37 via the conductive connection portion 36 a and the conductiveconnection portion 36 b respectively. This prevents the optical filter 1and the imaging device 15 from becoming electrically charged. As aresult, dust and lint are prevented from becoming adhered to the opticalfilter 1 and the seal glass 2 located near the focal plane of the takinglens 1.

Now, a discharge may occur in a camera in the prior art when its opticalfilter becomes electrically charged and the difference in the potentialoccurring between the optical filter and the photoelectric conversionelement increases to a certain degree. When such a discharge occurs,noise may be superimposed on a signal output by the photoelectricconversion element. In addition, depending upon the extent of thedischarge, the photoelectric conversion element itself may even bedestroyed. In contrast, since the optical filter 1 and the imagingdevice 15 are connected with each other with their potentials set equalto each other via the conductive area of the casing 37 in the camera inthis embodiment, there is no difference in the potential, therebyeliminating the problem described above.

In the explanation given above in reference to FIGS. 15, 16 and 17A-17D,the transparent electrodes 38A and 38B formed at the two surfaces of theoptical filter 1 and the transparent electrode 38C formed at the frontsurface of the seal glass 2 are both connected to the conductive area ofthe casing 37 in a continuous state. The camera that is to be explainednext differs from this structure in that the transparent electrodes38A-38C are connected with one another to achieve equal potentials andthat a voltage source is connected between the transparent electrodes38A-38C and the conductive area of casing 37. Thus, the explanation isnow given mainly on these differences. Since the other structuralfeatures are identical to those illustrated in FIGS. 15, 16 and 17A-17D,their explanation is omitted.

FIG. 18 is an enlargement similar to that in FIG. 16, which illustratesthe portion of the camera where the optical filter 1 and the imagingdevice 15 are provided. In FIG. 18, the same reference numbers areassigned to components identical to those illustrated in FIGS. 16 and17A-17D to preclude the necessity for an explanation thereof. While thetransparent electrodes 38A and 38B are connected to a terminal 50 a of avoltage source 50 via a conductive connection portion 36 aA, thetransparent electrode 38C is connected to the terminal 50 a of thevoltage source 50 via a conductive connection portion 36 b. A terminal50 b of the voltage source 50 is connected to the conductive area of thecasing 37. It is to be noted that while the voltage source 50 isillustrated as a DC source for purposes of achieving maximum conveniencein illustration, with the side on which the terminal 50 b is present setto (+), the present invention is not restricted to this example.Specifically, the potential resulting from an electrical charge variesdepending upon the material constituting the optical filter 1, and it isdesirable to adjust the polarity and the voltage at the voltage source50 to achieve the maximum effect in preventing an electrical charge incorrespondence to the specific type of material used.

In addition, the voltage generated by the voltage source 50 may be an ACvoltage instead of a DC voltage. If an AC voltage is generated by thevoltage source 50, its frequency should be set within the range ofapproximately several kHz to twenty kHz.

By adopting the structure described above, the potential generated bythe voltage source 50 is applied to the transparent electrodes 38A 38Crelative to the casing 37 to inhibit any electrical charge fromoccurring at the optical filter 1 and the imaging device 15.

FIG. 19 is an enlargement similar to that in FIG. 16, which illustratesthe portion of the camera where the optical filter 1 and the imagingdevice 15 are provided. In FIG. 19, the same reference numbers areassigned to components identical to those illustrated in FIGS. 16 and17A-17D to preclude the necessity for an explanation thereof. Inaddition, since the structural features other than that illustrated inFIG. 19 are similar to those illustrated in FIG. 15, their explanationis omitted.

When the movable members such as a blade 3 a and the like in the shutterunit 3 are constituted of non-conductive materials, static electricitymay be generated by the movement of the movable members to result in anelectrical charge occurring at the shutter unit 3 and the optical filter1 provided near the shutter unit 3. The structure illustrated in FIG. 19achieves prevention of an electrical charge from occurring at theshutter unit 3 and the optical filter 1 during the operation of theshutter unit 3.

A base plate 3 b of the shutter unit 3 is constituted of a conductivematerial such as aluminum, brass or plastic containing a carbon fiber. Alug plate (not shown), for instance, is connected to one end of a wire98 so that the wire 98 is screwed onto the base plate 3 b via the lugplate. Thus, the base plate 3 b and the wire 98 become electricallyconnected with each other. Another end of the wire 98 is connected to aterminal 60 a of a voltage source 60. While the transparent electrodes38A and 38B are connected to a terminal 60 b of the voltage source 60via a conductive connection portion 36 aA, the transparent electrode 38Cis connected to the terminal 60 b of the voltage source 60 via aconductive connection portion 36 b. It is to be noted that as explainedearlier in reference to FIG. 18, selections are made in regard to thepolarity of the voltage source 60, whether a DC voltage or an AC voltageis to be generated, the frequency of the voltage if an AC voltage isgenerated and the voltage level, to achieve an optimal state forpreventing the shutter unit 3, the optical filter 1 and the imagingdevice 15 from becoming electrically charged.

By adopting the structure described above, static electricity isprevented from being generated during the operation of the movablemembers in the shutter unit 3.

It is to be noted that while none of the shutter unit 3, the opticalfilter 1 and the imaging device 15 is connected to the conductive areaof the casing 37 in FIG. 19, the terminal 60 a or 60 b of the voltagesource 60 may be connected to the conductive area of the casing 37. Forinstance, by connecting the terminal 60 b to the conductive area of thecasing 37, static electricity that may be generated at the opticalfilter 1 and the imaging device 15 can be suppressed without having togenerate a voltage with the voltage source 60.

Now, foreign matter tends to enter the mirror box 72 (see FIG. 15)during a lens exchange or the like in a single lens reflex type DSCwhich allows taking lens exchange. Thus, it is desirable that the insideof the mirror box 72 be cleaned on a regular basis. During such acleaning process, foreign matter that has become adhered to the opticalfilter 1 and the like may not be readily removed even with air blown byusing a blower or the like. However, the camera explained above inreference to FIG. 19 facilitates the cleaning process as explainedbelow.

In FIG. 19, a taking lens detection unit 101 for detecting whether ornot the taking lens 20 (see FIG. 15) is attached, a cleaning modesetting switch 102 for setting the cleaning mode for the camera, arelease switch 103, a mirror actuator 104 for moving the mirror 12 (seeFIG. 15) vertically by interlocking with the photographing operation anda shutter actuator 105 for driving the shutter unit 3 are connected to aCPU 100 that controls the camera operation. This CPU 100 is furtherconnected with the voltage source 60.

The camera user removes the taking lens 20 from the camera main body 70,sets the cleaning mode for the camera by operating the cleaning modesetting switch 102 and turns on the release switch 103. In response tothis, the CPU 100 provides a control signal to the mirror actuator 104and the shutter actuator 105 to set the member 12 in a raised state,i.e., the state indicated by the 2-point chain line in FIG. 15 and toset the shutter unit 3 in an open state. Next, the CPU 100 provides acontrol signal to the voltage source 60 to cause the voltage source 60to generate a specific voltage. At this time, either an AC voltage or aDC voltage may be generated by the voltage source 60. This neutralizeselectrical charges at the shutter unit 3, the optical filter 1 and theimaging device 5 to reduce the force with which foreign matter adheresto the optical filter 1 and the imaging device 15 (the attractive forcegenerated by static electricity).

In addition, the foreign matter that is adhering to the optical filter1, the imaging device 15 and the like may itself be electricallycharged. In such a case, the foreign matter may be lifted off theoptical filter 1 or the imaging device 15 by generating an AC voltagewith the voltage source 60 or by applying a DC voltage having a polaritywhich will generate a repulsive force against the adhering foreignmatter. The foreign matter can be removed with ease by the camera userwith a blower or the like to blow air into the inside of the mirror box72 in the state in which the voltage is being generated by the voltagesource 60 as described above. At this time, the foreign matter can beremoved even more effectively by employing an apparatus thatelectrically charges the air blown out of the blower to generate anattractive force to attract the foreign matter with the electricallycharged air blown out of the apparatus.

After the cleaning process is completed as described above, the CPU 100interlocks with the camera user operation in which the release switch103 is turned on again to transmit a control signal to the voltagesource 60, the mirror actuator 104 and the shutter actuator 105. Thisstops the voltage generation at the voltage source 60, closes theshutter unit 3 and lowers the mirror 12.

The cleaning mode described above may be also adopted in the cameraexplained earlier in reference to FIG. 18. In addition, while the source50 in the camera explained in reference to FIG. 18 and the source 60 inthe camera explained in reference to FIG. 19 are provided inside thecameras, these voltage sources 50 and 60 may be omitted to supply avoltage from an external source provided outside the camera. In thatcase, the terminals 50 a and 50 b or the terminals 60 a and 60 b shouldbe set in a shorted state during normal operation. Then, when thecleaning mode is set, a voltage is applied by the external voltagesource to the terminals 50 a and 50 b or the terminals 60 a and 60 b. Byadopting this structure, a more compact, lighter and less costly camerais achieved with ease. At the same time, the removal of foreign matteris facilitated.

While the explanation given above in reference to FIGS. 15-19 uses anexample in which the present invention is adopted in an interchangeablelens type DSC, the present invention may also be adopted in a fixedtaking lens type DSC. In addition, while the explanation is given on anexample of a direct image forming type camera constituted by providingthe optical filter 1 and the imaging device 15 near the primary imageforming plane of the taking lens, the present invention may be adoptedin a DSC provided with a relay lens system as explained below inreference to FIG. 20.

FIG. 20 illustrates a schematic structure of an interchangeable lenstype DSC provided with a relay lens system comprising a field lens 42and a relay lens 45, with the same reference numbers assigned tocomponents identical to those in the interchangeable lens type DSCillustrated in FIG. 15 so that the explanation can be focused on thedifferences from the camera illustrated in FIG. 15.

The following explanation is given in reference to an assumed state,i.e., the photographing state in which the mirror 12 is as indicated bythe 2-point chain line in FIG. 20 and the shutter unit 3 is open. Thefield lens 42 is provided near the primary image forming plane of thetaking lens 20. Behind the field lens 42, mirrors 43 and 44 for bendingthe optical path are provided, and behind the mirror 44, the relay lens45 is provided. An image of the subject formed on the primary imageforming plane of the taking lens 20 is transmitted through the fieldlens 42, the mirror 43, the mirror 44, the relay lens 45 and the opticalfilter 1 to become reformed on the photosensitive surface of the imagingdevice 15 in a reduced size. In other words, the photosensitive surfaceof the imaging device 15 constitutes the secondary image forming planeof the taking lens 20. On an optical path “p” indicated by the 1-pointchain line in FIG. 20, a shadow will be cast on the subject image formedon the photosensitive surface of the photoelectric conversion element 15even by foreign matter such as dust and lint present near either theprimary image forming plane or the secondary image forming plane. Inorder to eliminate this concern, it is desirable to provide transparentelectrodes on the front surface of the field lens 12 and on the exitsurface of the relay lens 15 and to connect the transparent electrodesto the conductive area of the casing 37, the voltage source 50 (see FIG.18) or the voltage source 60 (see FIG. 19).

While the explanation has been given above on an example of applicationof the present invention in a camera, the present invention may beadopted in other optical apparatuses. The following is an explanation ofan example in which the present invention is adopted in an image inputapparatus (image scanner) given in reference to FIG. 21.

In FIG. 21, which illustrates a schematic structure of an image inputapparatus, an image input unit 250 comprises a mirror 162, an imageforming lens 164, and imaging device 252, a housing 258 for housingthese members and the like. At the imaging device 252, pixels arearrayed along a single column or over a plurality of columns along thedirection extending perpendicular to the page on which FIG. 21 isprinted. A ribbed belt (timing belt) 158 is provided connecting betweenwheels 154 and 156. The image input unit 250 is secured at the ribbedbelt 158. The wheel 154 is driven to rotate by a stepping motor 152 todrive the image input unit 250 in the horizontal direction relative tothe page on which FIG. 21 is printed in a reciprocal movement. Thesecomponents are housed in a casing 172. A platen glass 166 is provided atan opening at the top of the casing 172, and a document holder 168 whichis capable of covering the entire platen glass 166 is further provided.

A host computer (not shown) is connected to the image input apparatus150 structured as described above. In response to an operation of thehost computer by the operator after he sets a document M to be read onthe platen glass 166, the host computer issues an image input command tothe image input apparatus 150. In response to this image input command,the image input apparatus 150 starts an image input of the document Mand transfers the image data to the host computer. In other words, anoperation in which an image of the document M formed by the imageforming lens 164 is linearly read by the imaging device 252 and then theimage input unit 250 is made to move in the horizontal directionrelative to the surface of the page on which FIG. 21 is printed at aspecific moving pitch is repeated to input a two-dimensional image ofthe document M.

During this process, if foreign matter is adhering to the front surface(document mounting surface) or the rear surface of the platen glass 166,the exit surface of the image forming lens 164 or the photosensitivesurface of the imaging device 252, the shadow of the foreign matter willbe transferred. In particular, if foreign matter adheres to the exitsurface of the image forming lens 114 or the photosensitive surface ofthe imaging device 252, a shadow will be cast constantly on the imageinput at specific pixels of the imaging device 252 a line will betransferred onto the input image resulting in a faulty picture.

To deal with this problem, a transparent electrode (not shown) is formedat the rear surface of the platen glass 166 in the image input apparatusillustrated in FIG. 21. This transparent electrode is connected to anelectrically conductive area of the casing 172 via a conductiveconnection portion 170. Likewise, a transparent electrode (not shown)formed at the exit surface of the image forming lens 164 and atransparent electrode (not shown) formed at the front surface of a sealglass 252 a of the imaging device 252 are respectively connected to theelectrically conductive area of the housing 258 via a conductiveconnection portion 256 and a conductive connection portion 254. Thestructure illustrated in FIG. 17B or FIG. 17C, for instance, may beadopted in the conductive connection portions 170, 256 and 254.

The electrically conductive area of the housing 258 and the electricallyconductive area of the casing 172 are connected with each other by aconductive member 260 which can be deformed freely, such as a curled orslack flexible printed circuit board (FPC). As a result, even when theimage input unit 250 moves in the left and right direction on the pageon which FIG. 21 is printed, the electrically continuous state can bemaintained for the electrically conductive area of the housing 258 andthe electrically conductive area of the casing 172. By adopting thestructure explained above, in which the platen glass 166, the exitsurface of the image forming lens 164 and the imaging device 252 are allconnected with the casing 172 with their potentials set equal to that ofthe casing 172, generation of static electricity is suppressed. Thus, itbecomes possible to prevent foreign matter from adhering to the platenglass 166, the image forming lens 164 or the imaging device 252 to casta shadow on the input image.

The present invention may be adopted in optical apparatuses other thanthose in the examples explained above. For instance, it may be adoptedin an image input apparatus without an image forming lens, which readsthe document to be read by placing the imaging device in close proximityto the document or an image input apparatus that is provided with alight guide constituted in the form of a fiber scope between the objectof image input and the imaging device.

1. An optical filter comprising: a first birefringent plate constitutingan optical element that spatially divides incident light along a firstdirection extending perpendicular to a direction in which the incidentlight advances to achieve two separate light fluxes; a vibrational planeconverting plate that changes the vibrational plane of each of the twolight fluxes emitted from said first birefringent plate; and a secondbirefringent plate constituting an optical element that spatiallydivides each of the two light fluxes emitted from said vibrational planeconverting plate into two light fluxes along a second direction which isdifferent from said first direction, to achieve a total of four separatelight fluxes, wherein: at least either said first birefringent plate orsaid second birefringent plate is constituted of a material with alarger difference between the extraordinary ray refractive index and theordinary ray refractive index compared to that of quartz.